Plasma LH and FSH concentrations in prepubertal beef heifers beforeand inresponse to repeated injections of low doses of Gn-RH. B. J. McLeod, W. Haresign, ...
Plasma LH and FSH concentrations in prepubertal beef heifers before and in response to repeated injections of low doses of Gn-RH B. J.
McLeod, W. Haresign, A. R. Peters and G. E. Lamming
A.R.C. Research Group on Hormones and Farm Animal Reproduction, University of Nottingham, School of Agriculture, Sutton Bonington, Loughborough, Leicestershire LE12 5RD, U.K.
Summary. Blood samples were taken at 15-min intervals from prepubertal beef heifers, 6 aged 4 months and 6 aged 10 months, to determine plasma LH and FSH concentrations. Two animals of each age group were then subjected to 9 consecutive injections of 0\m=.\5,2\m=.\0or 5\m=.\0\g=m\gGn-RH at 2-h intervals. LH episodes were evident in all animals before treatment, and in 11/12 heifers occurred at a rate of between 1 and 4 per 24 h. One 10-month old heifer, later detected in oestrus, displayed 10 episodes/24 h. Mean FSH concentrations did not differ significantly between age groups. In all animals, Gn-RH injections induced an episodic pattern of LH secretion which was
significantly different between Gn-RH doses,
but not between age groups. A distinct to each Gn-RH injection in only in FSH concentrations occurred episode-like response one heifer. These results provide further evidence that the prepubertal period is associated with a lack of pituitary stimulation, rather than an inability to respond to
Gn-RH. Introduction There appear to be a number of similarities in the endocrine control of the final stages of follicle development in the sheep and the cow, which are reflected in the patterns of episodic LH secretion. Maturation of the preovulatory follicle in both species is associated with a marked increase in the frequency of LH episodes (Baird, 1978; Rahe, Owens, Fleeger, Newton & Harms, 1980). In addition, periods of ovarian inactivity—seasonal anoestrus in the ewe and post-partum anoestrus in the cow—are associated with a low frequency of LH episodes (Lamming, Wathes & Peters, 1981 ; McLeod, Haresign & Lamming, 1982a, b). Increasing this frequency by repeated administration of low doses of exogenous Gn-RH will induce follicular growth, culminating in a normal ovarian cycle in both species (Riley, Peters & Lamming, 1981; McLeod et al, 1982a, b). The role of FSH in preovulatory follicle development is less well defined. Schams, Schallenberger, Hoffman & Karg (1977) reported the occurrence of an FSH peak around Day 18 of the oestrous cycle of the cow, which they suggested was a stimulus for follicular growth. However, other workers have failed to detect such an increase (Akbar, Reichert, Dunn, Kaltenbach & Niswender, 1974; Butler, Katz, Arrióla, Milvae & Foote, 1983). In addition, no relationship is apparent between plasma FSH concentrations and the resumption of ovarian cycles post partum (Schams et al, 1978; Webb, Lamming, Haynes & Foxcroft, 1980). It has also been demonstrated that an abrupt increase in the frequency of LH episodes is a prerequisite for the onset of puberty in the sheep, and that precocious puberty can be induced by repeatedly injecting purified LH to increase episode frequency artificially (Ryan & Foster, 1980). A similar relationship may exist in the heifer; a pulsatile pattern of LH release is evident in 0022-4251 /84/010137-08S02-00/0 © 1984 Journals of Reproduction & Fertility Ltd
prepubertal heifers from as early as the first month of life, with episode frequency increasing with age (Schams, Schallenberger, Gombe & Karg, 1981). Long-term changes in basal LH concentrations have also been reported in prepubertal and pubertal animals. In both the heifer and the ewe lamb, basal plasma LH levels are higher and more variable during the prepubertal period than after the onset of ovarian cyclicity (González-Padilla, Wiltbank & Niswender, 1975; Ryan & Foster, 1980; Kiser et al, 1981). Schams et al. (1981) reported episodic release of FSH in the prepubertal heifer, with episode frequency increasing with age. However, plasma FSH levels have also been reported to remain relatively constant during the prepubertal and pubertal period in heifers (González-Padilla et al, 1975) and ewe lambs (Foster, Lemons, Jaffe & Niswender, 1975). It has not yet been determined whether the low LH episode frequency observed in prepubertal animals is due to an inherently poor response of the juvenile anterior pituitary to endogeneous Gn¬ RH, or to a low frequency of endogenous Gn-RH secretion. The objective of this trial, therefore, was to monitor endogenous LH and FSH secretion in prepubertal heifers of two ages, and then to assess
their response to low doses of exogenous Gn-RH.
Materials and Methods Animals and management. Hereford Friesian heifers, 6 aged approximately 4 months (Heifers ¬ , mean ± s.e.m. liveweight 111-7 + 3-5 kg), and 6 aged approximately 10 months (Heifers 7-12, mean + s.e.m. liveweight 208-3 + 8-8 kg), were housed indoors under conditions of natural and and observed twice daily for oestrus, from 2 weeks before, until 6 weeks daylength temperature after treatment. All animals were fed concentrates and hay twice daily, with fresh water always
available.
Treatment and blood
sampling. The heifers of each age were randomly assigned to three equal size. After the insertion of a jugular vein catheter 24 h earlier, blood samples were collected from all animals at 15-min intervals for 24 h to monitor endogenous LH and FSH patterns. Immediately thereafter, each heifer was given 9 successive i.v. injections of 0-5 pg (Group 1, Heifers 1, 2, 7 & 8), 2-0 pg (Group 2, Heifers 3,4,9 & 10) or 5-0 pg (Group 3, Heifers 5, 6, 11 & 12) Gn-RH (Lutai: Fabwerke Hoechst AG, Frankfurt, West Germany) in 2 mi sterile saline (9 g NaCl/1) at 2-h intervals. Blood sampling at 15-min intervals was continued until 2 h after the final Gn-RH injection. Throughout the treatment period the blood samples were collected immediately before each Gn-RH injection. LH assay. Plasma LH concentrations were measured by the double-antibody radioimmuno¬ assay method of Webb, Lamming, Haynes, Hafs & Manns (1977). The limit of sensitivity of the assay within this study was 0-5 ng NIH-LH-B9 equiv./ml plasma, and the inter- and intra-assay coefficients of variation were 12-5% and 5-6% respectively. There was negligible cross-reaction with other protein hormones. Plasma LH concentrations. An LH episode was defined by the criteria previously described (Riley et al, 1981 ; McLeod et al, 1982b). The maximum LH concentrations attained and the area under each LH episode were used to compare differences in LH secretion between age of heifers during the pretreatment period, and in response to Gn-RH injections. The area under an LH episode included only that area above the mean pretreatment basal concentration (i.e. the mean of all values excluding the endogenous LH episodes). FSH assay. Blood samples were assayed for FSH by the heterologous radioimmunoassay of Webb et al (1980). Within this study the limit of sensitivity of the assay was 10-8 ng NIH-FSH-B1 equiv./ml plasma and the inter- and intra-assay coefficients of variation were 12-4% and 8-2% respectively. Statistical analysis. The data were analysed by analysis of variance, 2 analysis or Student's t test, as appropriate. treatment groups of
Results
Plasma LH profiles over the 24-h pretreatment period are shown in Text-fig. 1. Mean basal LH concentrations (excluding LH episodes) ranged from 1-34 to 2-66 ng/ml, but were not significantly different between animals or between age groups (see Table 1). LH episodes were evident in all animals and, with the exception of one heifer, occurred at a frequency of 1-4 episodes/24 h. In the remaining animal (Heifer 7) 10 LH episodes were recorded. This animal was detected in oestrus 10 days after the intensive blood sampling period, whereas none of the other 11 heifers displayed oestrus within the ensuing 6 weeks. Although individual LH episodes differed markedly in both the maximum LH concentration attained (range 3-0-12-3 ng/ml plasma) and the area under the peak (range 20-172 units), there were no significant differences between animals or between age groups in the mean frequency, amplitude or area of the endogenous LH episodes. Plasma FSH profiles for individual heifers are shown in Text-fig. 2. Mean plasma FSH concentration, over the 24-h pretreatment period varied widely between animals, ranging from 21-6 to 1171 ng/ml, but there were no significant differences between the two age groups (Table 1). Although FSH concentrations fluctuated throughout the 24-h pretreatment period in some animals, no circadian patterns were apparent and in other animals plasma concentrations remained relatively constant.
Heifer 1
Heifer 7
(b)
\*/S
